Page 18 - Issue 3 2023

TECHNICAL

Identifying ageing and installation

issues in an HV bushing

The 69 kV bushings on a 1969-vintage 10 MVA, 69/13,09 kV Dyn1 two-winding transformer were
subjected to some important electrical tests following the completion of on-load tap changer
(OLTC) maintenance on this transformer.

Information from Megger

his article describes the tests and
discusses the test results. It shows
Tthe value of line frequency power
factor testing when used as the primary
approach to evaluating HV insulation.

Overall transformer line
frequency power factor (LF PF)
tests
Losses measured for the low-to-ground
(CLG) and high-to-ground (CHG) Table 1: Overall transformer LF PF test results – initial condition
insulation systems were higher than
those measured for the interwinding, temperature corrected LF PF test results for bushings H1 and H2 earned the bushings
or high- to-low (CHL), insulation system good (G) ratings while the individual temperature corrected LF PF test result for bushing
of the transformer. The temperature- H3 resulted in an ageing (A) assessment.
corrected (i.e. 20°C equivalent) LF PF The test results also display an interesting anomaly in the ‘individual temperature
test results for CLG and CHG insulation correction’ values. A temperature-correction (TC) factor is used to determine a test
components were within acceptable specimen’s 20°C equivalent LF PF value when the specimen is power-factor tested at a
limits (<0,5%), and even within limits non-20°C temperature.
established for new transformers. An ITC factor is a TC factor that is unique to every test specimen as it is based on the
However, the CHG LF PF test result specimen’s specific condition. Note that the ITC factor for bushing H3 (~0.6) is different
(0,43%) was approximately 1,8 times the than the ITC factors determined for bushings H1 and H2 (~1,04). The non-uniformity
CHL LF PF result (0,24 %) (Table 1). that exists in these ITC factors is a clear indication that the H3 bushing has a different
Based on recent experiences using insulation condition than H1 and H2 bushings.
Narrowband Dielectric Frequency
Response (NB DFR) testing and successes Transformer bushing C1 test investigation
in finding hidden issues not observed ITC factors are far more accurate than TC factors accessed in a look-up table. To
by LF PF tests, a NB DFR test was carried underscore the problem with temperature-correction look-up tables, consider the
out. A NB DFR test is the measurement following:
of % PF or % DF at different frequencies • If the end-user had relied on correction tables, the TC factor would have been
from 1 Hz to 505 Hz. determined based upon the average of the ambient and bushing temperatures. This
In addition, the testing specialist
performed 10 kV C1 LF PF tests on the
transformer’s high-side winding bushings
(Table 2). The H3 bushing produced
elevated LF PF test results (measured
and temperature-corrected) that were
notably greater than results for the
sister bushings mounted on the same Table 2: C1 test results for the high-side bushings, including 10 kV LF PF and 250 V NB DFR results,
transformer. Accordingly, the individual measured and temperature-corrected via the ITC method

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